1. Field of the Invention
The present invention relates to droop correction for laser elements generated during image formation in a multi-beam image forming apparatus that forms images using plural semiconductor laser elements serving as an optical beam generation unit.
2. Description of the Related Art
In electrophotographic apparatuses such as laser printers and digital copiers, electrostatic latent images corresponding to recording information are formed by an optical beam generation unit using laser beams after a photoconductive drum is uniformly charged. Then, the electrostatic latent images are developed with toner and transferred onto a sheet by a transfer unit and further fixed so as to form images.
FIG. 1 shows the schematic configuration of a multi-beam image forming apparatus. A photosensitive drum 700, on which a toner image is to be formed, is uniformly charged by a charging unit 701 and then exposed to a laser beam from an optical scanning unit 702 modulated by image data transmitted from a host unit 707. Accordingly, an electrostatic latent image is formed on the photosensitive drum 700. Next, the latent image on the photosensitive drum 700 is developed by a development unit 703 to form a toner image. The toner image formed on the photosensitive drum 700 is transferred onto a print sheet 708 by a transfer unit 704. The print sheet 708 onto which the toner image is transferred is conveyed to a fixing unit 705 where the toner image is fixed on the print sheet 708. Furthermore, the remaining toner on the photosensitive drum 700, which has not been transferred onto the print sheet 708 by the transfer unit 704, is removed by a cleaning unit 706.
Conventionally, as an image forming apparatus of this type, a multi-beam image forming apparatus has been proposed that scans plural lines at the same time with plural laser beams through a polygon mirror or the like to form an image. Such a multi-beam image forming apparatus has the characteristic of performing high-speed image formation using a polygon motor that rotates at low speed and a low-power semiconductor laser because it forms a plural-line image using one surface of a polygon mirror.
In order to generate multi beams, some methods are known. According to one method, plural semiconductor lasers are optically synthesized together to generate plural laser beams. Furthermore, a method using a semiconductor laser array is known in which plural semiconductor laser elements are arranged in series so as to be packaged. Furthermore, a method using a surface light-emission laser is known in which plural semiconductor laser elements are two-dimensionally arranged so as to be packaged. Among the above methods, the method in which the plural lasers are optically synthesized together causes a complex configuration due to accuracy in scanning positions and further generation of multi beams. The method using the laser array is advantageous in terms of accuracy in the arrangement of the semiconductor laser elements as multi beams are further generated. However, the semiconductor laser array itself generates heat due to the heat generated when the respective semiconductor laser elements emit light because it is constituted by the plural semiconductor laser elements. As a result, the emission light amounts of the respective semiconductor laser elements may be varied. This phenomenon is called droop.
FIG. 2 shows an example of the variation in the light amount due to the droop. The light amount of the semiconductor laser element is set to a basic light amount P0 used in an image forming range by an APC (Auto Power Control) circuit as a known art. When solid printing having a toner covering rate of 100% is performed as shown in FIG. 2, the semiconductor laser element continuously emits light. Therefore, the heat generation amount of the semiconductor laser element is increased. Due to this large self heat generation, the actual light amount gradually reduces relative to the light amount P0 set by the APC (droop phenomenon). Therefore, the variation in the light amount ΔP is generated between scanning start and finish points, which results in degradation in image quality due to density irregularities.
As an example for correcting the droop, Patent Document 1 pays attention to specific pixels to be printed and generates a light-emission-level correction signal based on the light emission time of a semiconductor laser and the previous n pixel data.
Patent Document 1: JP-A-9-314908
However, this correction method requires an extremely high-speed and high-performance calculation unit because it performs a calculation in which a correction signal is generated for every pixel. In recent color printing and the like, the amount of print data information itself has been huge and high-speed output is now being demanded. Therefore, the above method is not necessarily practical from the viewpoint of cost performance.
FIG. 3 shows an example of the variation in the light amount due to the droop in a semiconductor laser array. Here, a 5-channel optical scanning unit is employed that scans image data by using a 5-element semiconductor laser array in which five semiconductor laser elements are one-dimensionally arranged in series. As shown in FIG. 3, attention is paid to the variation in the light amount of the semiconductor laser element corresponding, for example, to a CH (channel) 3. The light amount of the semiconductor laser element of the CH 3 is set to the light amount P0 by the APC, which becomes the light amount of CH3 used in an image forming range.
In the image forming range, the semiconductor laser element of the CH3 emits light at the light amount P based on image data. With the self heat generation of the semiconductor laser element of the CH3, the semiconductor laser array generates heat, which results in the gradual reduction of the light amount of the semiconductor laser element of the CH3 (as shown in dotted lines in FIG. 3). When the semiconductor laser elements of a CH2 and a CH4 emit light after a predetermined time elapses, the heat generation amount of the semiconductor laser array is further increased while the light amount of the semiconductor laser element of the CH3 continues to be reduced. Then, when a CH 1 and a CH 5 emit light, the light amount of the CH3 is further reduced. At last, the light amount of the CH3 is reduced by an amount of ΔP1 relative to the light amount P0, and printing per scanning is completed. Due to this large light-amount variation, a variation in image density is caused in the image forming range.
The above-described droop is related to the light emission time of the respective semiconductor laser elements, and its influence becomes the greatest when the respective semiconductor laser elements continuously emit light. Besides the continuous light emission, the accumulation of the light emission time leads to the accumulation of heat even when the emission/extinguishment of light is repeatedly performed. In this case also, the influence due to the droop is caused.
FIG. 4 schematically shows an example of an image pattern in which the influence due to the droop is easily caused. When image data in which solid printing and dots are continuously formed at the first scanning are printed, a laser light amount is reduced due to continuous light emission in a solid printing region as described above. Therefore, when the dots are printed at a region (a), it is not possible to perform printing with a predetermined light amount, and as a result, printed dots would become less dense. Furthermore, when the dots are printed at a region (b) where light is not first emitted at the next scanning, it is possible to perform printing with the predetermined light amount. As a result, density irregularities are caused at the dots due to a difference in density between the dots after the continuous light emission is performed at the solid printing region and the dots next to a non-printing region, which in turn causes degradation in image quality.